Teach Lint/AssignmentInCondition about case

[sambostock]

This teaches Lint/AssignmentInCondition about case, to match the behavior with other conditionals.

 case test = 10
+          ^ Use `==` if you meant to do a comparison or wrap the expression in parentheses to indicate you meant to assign in a condition.
 when nil
 end

case (test = 10)
when nil
end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

This cop emulates the following Ruby warning:

$ ruby -ce 'if (test = 10); end'
-e:1: warning: found `= literal' in conditional, should be ==
Syntax OK

It does not display the warning for case:

$ cat example.rb
case (test = 10)
when nil
end

$ ruby -wc example.rb
Syntax OK

[koic]

Ah, I made a mistake in my previous comment about Lint/LiteralAssignmentInCondition 😓

However, case...when differs from if and while, as it does not independently form a condition. This might call for an initial discussion in the style guide.
https://rubystyle.guide/#safe-assignment-in-condition